It is well known to use a subsurface safety valve as disclosed in U.S. Pat. Nos. 3,782,461 and 4,161,219 which is actuated to the open position by the application of hydraulic fluid from the well surface and which is moved to the closed position by biasing means such as an enclosed pressurized gas chamber or a mechanical spring. Hydraulic force is applied to a piston and cylinder assembly and acts against the biasing force of the pressurized gas charge or spring in order to open and hold the safety valve opened. When the hydraulic pressure from the well surface is reduced below a certain value the biasing force acts to close the valve.
However, since the valve is a safety valve, it is imperative that it must close under all circumstances and therefore the biasing closing force must be positive and reliable. In the past, a biased mechanical spring acting to close against a hydraulic piston has been the standard. However, as valves are set deeper in the well pressurized chambers containing compressed inner gas have become the norm. In concept, the gas acts against a piston area to create a closing force much higher than that obtainable with a conventional mechanical spring. However, a failsafe closing safety valve has been more difficult to provide using compressed gas chambers. Seal leakage or failure may occur in the safety valve allowing the compressed biasing gas pressure to escape or a seal may leak allowing high pressure tubing gas to act against and overcome the biasing gas chamber. In such cases, the safety valve will fail to close and cannot accomplish its sole function.
The present invention is directed to a failsafe safety valve utilizing a pressurized gas chamber as a biasing closing force in which an equalizing system is provided for equalizing fluid pressure on opposite sides of the piston and cylinder actuating assembly. In the event of a failure of a seal in the piston and cylinder assembly a small biasing spring can easily close the equalized valve.